1. Field of the Invention
The present invention relates to an optical scanning device and an image forming apparatus in which the optical scanning device can be used.
2. Description of the Related Art
Conventionally, there is known an optical scanning device that deflects a light flux with a deflector such as an optical deflector, focuses the deflected light flux that is in the form of a microscopic spot light on a surface to be scanned, and scans the surface in a main scanning direction at a constant speed with the microscopic spot light by rotation of the deflector. The optical scanning device is used as a latent-image writing unit or the like in an image forming apparatus such as a laser beam printer, a laser beam plotter, a facsimile device, and a digital copier. The optical scanning device deflects and reflects a laser beam emitted from a laser light source by the optical deflector, to scan the surface of an image carrier or the like with the laser beam. At the same time, the optical scanning device writes an image onto the surface of an image carrier or the like by modulating an intensity (e.g., ON, OFF) of the laser beam according to image signals.
A rotating polygon mirror (polygon scanner) that is made to rotate at a constant speed is widely used as the optical deflector. However, the rotating polygon mirror is bulky and it is mechanically rotated at a high-speed. These facts lead to generation of banding due to oscillation, rise in temperature, generation of noise, and an increase in power consumption. Meanwhile, by using a micromirror, as a deflector of the optical scanning device, that has a resonance structure using a micromachining technique and sinusoidally oscillates, the device is downsized, and the banding due to the oscillation, the rise in temperature, the noise, and the power consumption can be largely reduced.
The micromirror (oscillating mirror) is superior as the deflector as mentioned above. However, with a micromirror it is difficult to optically and simultaneously correct both a deviation between image heights of a main-scanning beam spot diameter and a main-scanning position error. A technology for optically correcting the deviation between the image heights of the main-scanning beam spot diameter and electrically correcting the main-scanning position error to improve image quality is disclosed in Japanese Patent Application Laid-open No. 2002-258204 and Japanese Patent Application Laid-open No. 2007-086335. However, it is known that if a rotation angle of the micromirror is increased to widen a field angle in order to ensure a predetermined scan range, the main-scanning position error becomes prominent, and that even when electrical correction is performed, there occurs degradation in image quality such as uneven density due to a residual error. However, Japanese Patent Application Laid-open No. 2002-258204 and Japanese Patent Application Laid-open No. 2007-086335 do not discuss this matter. In addition, the scan range is divided into areas and a scanning position error is corrected for each divided area. In this case, if a division number (i.e., the number of areas in which the scan range is divided) is increased to narrow the areas, occurrence of the residual error can be reduced. However, in this approach, it is necessary to prepare a variable table for the division number, and thus, an increase in the division number leads to an increase in cost.
The electrical correction of the main-scanning position error taught by the technology described in Japanese Patent Application Laid-open No. 2002-258204 and some problems about the technology will be explained below.
Because the oscillating mirror uses a resonance phenomenon and thereby sinusoidally oscillates, scanning speeds in the image heights are different from each other. For this reason, it is necessary to adjust dot positions in the main scanning direction. Therefore, dot position correction in the main scanning direction is concurrently performed as follows. FIG. 10 is a diagram for explaining a correction amount of a main scanning position in each pixel in the main scanning direction when the oscillating mirror is driven by a signal with a single frequency. The main-scanning area is divided into a plurality of areas, e.g., eight areas, and the number of phase shifts is set for each area and the displacements in the areas are corrected in a stepwise manner so that the overall displacement in the main-scanning direction becomes zero at each boundary between the areas by means of broken-line approximation.
For example, assuming Ni to be the number of pixels in “i” area, a shift amount of each pixel to be ⅛ units of a pixel pitch p, and ΔLi to be a displacement between main-scanning reach positions at both ends of each area, the following equation is obtained:ni=Ni/(ΔLi/(p/8))and, a phase is simply shifted for each ni pixel. Let fc be a pixel clock, then a phase difference Δt as a total becomes as follows by using the number of times of phase shift Ni/ni:Δt= 1/16fc×∫(Ni/ni)di and, a phase difference at an N-th dot pixel can also be set using the number of accumulations of the phase shift performed so far.
If the shift amount at each pixel is increased, the difference between pixels is easily noticeable on an image, and thus, the shift amount is desirably set to ¼ units or less of the pixel pitch p. Conversely, if the phase shift amount is decreased, the number of phase shift times is increased, which results in an increase in memory usage. Each width of divided areas may be equal to each other or may not be equal to each other. As shown in FIG. 10, by setting an area width of an area with a large main-scanning position error to be narrow and an area width of an area with a small main-scanning position error to be wide, it is possible to efficiently correct the main-scanning position error.
It is known that, by increasing the division number it is possible to further decrease the residual error after correction of the main-scanning position error. FIG. 11 represents a situation where the division number is 16. Specifically, FIG. 10 represents a situation where the division number is 8 (a, b, c, d, e, f, g, h) and FIG. 11 represents a situation where the division number is 16 (a1, a2, b1, b2, c1, c2, d1, d2, e1, e2, f1, f2, g1, g2, h1, h2). It is found that the residual error in case of 16 divisions is smaller than that in 8 divisions. In this manner, if the division number is increased, the residual error after correction in the main-scanning position error is reduced. However, if the division number is smaller, the small memory capacity is sufficient for each variable table with correction data in each divided area, which enables each variable table to be configured with low cost.
As explained above, to correct the main-scanning position error caused by the oscillating mirror, the division number needs to be increased and it is thereby difficult to realize the correction at a low cost. Such a problem as above is similar to that in the technology described in Japanese Patent Application Laid-open No. 2007-086335. There is a problem so far that it is difficult to correct the main-scanning position on the surface to be scanned with low cost and high precision when the oscillating mirror that sinusoidally oscillates is used, and that a formed image is low in quality.
The oscillating mirror also has a problem that its scanning characteristics as a single unit vary greatly with time.